The present invention relates to systems for air cooling a compartment or air conditioning systems as they are commonly known, and in particular, relates to systems for air conditioning the cab or occupant compartment of a vehicle. Systems of this type generally employ a compressor/pump for compressing a refrigerant gas which is condensed in a heat exchanger and circulated to a second heat exchanger located in the occupant compartment for evaporation therein, and cooling of the surrounding air in the occupant compartment. Circulation of the condensed refrigerant to the second heat exchanger or evaporator as it is commonly called is controlled by an expansion control valve provided in the flow system between the condensing heat exchanger and the evaporator.
Heretofore, expansion control valves have controlled the flow of refrigerant by providing a means responsive to the evaporator superheat to control movement of a movable valve member. Known expansion control valves typically employ a diaphragm exposed to refrigerant pressure leaving the valve. A bulb senses the temperature at the evaporator outlet and in response thereto pressurizes a fluid filled chamber acting on the opposite side of the diaphragm. Movement of the diaphragm in response to the pressure differential thereacross is employed to move a refrigerant flow control valve member. Such pressure responsive refrigerant expansion control valves thus function to control the flow of liquid refrigerant to the evaporator in response to demand, or rate of vaporization in the evaporator.
Pressure responsive refrigerant flow control valves have found wide spread usage in automotive air conditioning systems because of their simplicity and reliability. However, such pressure responsive valves are limited in their control capability by virtue of their responsiveness to only the refrigerant superheat at the evaporator outlet. Thus, the known systems for controlling refrigerant flow in air conditioning systems, require a pressure sensing means in the evaporator fluid flow circuit capable of sensing the differential pressure.
Recent trends in automotive design have dictated the need for electrical control of not only engine operation, but also auxiliary systems on the vehicle which derive their power from the engine. Thus it has been desired to provide an all electrically controlled air conditioning system for the vehicle, and particularly, a system which could integrate the control of the electric compressor clutching mechanism and the refrigerant flow control in a common microcomputer employed for engine operating control. It has thus been desired to find a way or means of electrically controlling the flow of refrigerant in the automotive air conditioning system in order that the flow of refrigerant may be determined by parameters other than superheat of the evaporator. For example, where the rate of change of temperature in the vehicle occupant compartment is high or rapid, it may be desirable to attenuate the flow of refrigerant in anticipation of overcooling once the desired compartment temperature has been reached.
A known system for providing electronic control of refrigerant flow in a refrigeration system is that shown and described in U.S. Pat. No. 4,571,951, which teaches an electrically controlled refrigerant flow control valve, which is controlled in response to the measurement of pressure in the refrigerant flow circuit at a point near the evaporator discharge port. The flow pressure is then mathematically converted to a saturation temperature and is compared with a measured flow temperature at the evaporator outlet port and the temperatures are compared for providing an electrical control signal to the flow control valve operator.